This research program combines the resources of the Department of Medicine and Radiology of the Children's Hospital Medical Center and Dr. David Baltimore's laboratory in the Department of Biology of the Massachusetts Institute of Technology in a center of laboratory and clinical research into the problem of childhood leukemia. The clinical research deals with the therapeutic indications for the total body radiation and bone transfusion in end stage leukemia and in non malignant bone marrow failure. The research also investigates techniques for replacement of phagocytes in patients with severe neutropenia. The laboratory research program includes investigations of reverse transcriptase in mammalian cells, an approach to clonal selection of leukemia cells in vitro, examination of specific targets of cancer chemotherapeutic agents, the localization of transforming virus genes in specific transcription systems and the effect of cell fusion on the immunogenicity of tumor antigens.